diff --git a/skills/browser-trace/package.json b/skills/browser-trace/package.json
index 566b63c..1327023 100644
--- a/skills/browser-trace/package.json
+++ b/skills/browser-trace/package.json
@@ -2,5 +2,8 @@
   "name": "browser-trace",
   "version": "0.1.0",
   "private": true,
-  "type": "module"
+  "type": "module",
+  "scripts": {
+    "test": "node --test scripts/*.test.mjs"
+  }
 }
diff --git a/skills/browser-trace/scripts/snapshot-loop.mjs b/skills/browser-trace/scripts/snapshot-loop.mjs
index 00b297b..e4bd4d2 100755
--- a/skills/browser-trace/scripts/snapshot-loop.mjs
+++ b/skills/browser-trace/scripts/snapshot-loop.mjs
@@ -4,62 +4,160 @@
 //
 // Each tick opens a one-shot CDP connection via `browse --ws <target> ...`
 // (bypasses the `browse` daemon so it doesn't fight the main automation).
+//
+// Lifecycle: stop-capture sends SIGTERM and then waits up to ~3 seconds
+// before falling back to SIGKILL. The loop must therefore wake from its
+// inter-iteration sleep promptly when SIGTERM arrives, otherwise long
+// `interval-seconds` settings cause SIGKILL to fire mid-iteration and the
+// run loses its last DOM/screenshot pair.
 
 import fs from 'node:fs';
 import path from 'node:path';
+import { fileURLToPath } from 'node:url';
 import { spawnSync } from 'node:child_process';
 
-import { isoStampForFilename, sleepMs } from './lib.mjs';
+import { isoStampForFilename } from './lib.mjs';
 
-const [target, RD, intervalArg] = process.argv.slice(2);
-if (!target || !RD) {
-  console.error('usage: snapshot-loop.mjs <target> <run-dir> [interval-seconds]');
-  process.exit(2);
-}
+// Per-call timeout for `browse --ws ...` invocations. A hung browse CLI
+// would otherwise block this loop indefinitely until the parent SIGKILL
+// arrives, leaving the run truncated. Tunable via env so tests / heavy
+// pages can extend it.
+const SNAPSHOT_TIMEOUT_MS = Number(process.env.O11Y_SNAPSHOT_TIMEOUT_MS) || 30_000;
 
-const intervalMs = (Number(intervalArg) || 2) * 1000;
-const indexPath = path.join(RD, 'index.jsonl');
+// Run as a CLI only when invoked directly. Letting the module be imported
+// (e.g. from snapshot-loop.test.mjs) keeps the stop-signal helper unit-
+// testable without booting the full sampler loop.
+const isEntry = (() => {
+  if (!process.argv[1]) return false;
+  try {
+    return fileURLToPath(import.meta.url) === path.resolve(process.argv[1]);
+  } catch {
+    return false;
+  }
+})();
+if (isEntry) await runSampler();
 
-let stopping = false;
-process.on('SIGTERM', () => { stopping = true; });
-process.on('SIGINT',  () => { stopping = true; });
+async function runSampler() {
+  const [target, RD, intervalArg] = process.argv.slice(2);
+  if (!target || !RD) {
+    console.error('usage: snapshot-loop.mjs <target> <run-dir> [interval-seconds]');
+    process.exit(2);
+  }
 
-while (!stopping) {
-  const ts   = isoStampForFilename();
-  const png  = path.join(RD, 'screenshots', `${ts}.png`);
-  const html = path.join(RD, 'dom',         `${ts}.html`);
-  const tmp  = `${html}.partial`;
+  const intervalMs = (Number(intervalArg) || 2) * 1000;
+  const indexPath = path.join(RD, 'index.jsonl');
 
-  // Best-effort screenshot. If browse fails we just don't get one this tick.
-  spawnSync('browse', ['--ws', target, 'screenshot', png], { stdio: 'ignore' });
-  if (fs.existsSync(png) && fs.statSync(png).size === 0) {
-    fs.unlinkSync(png);
-  }
+  // Single shared stop signal. SIGTERM/SIGINT both flip `stopping` and
+  // resolve the promise so any in-flight wait can short-circuit.
+  const stop = createStopSignal();
 
-  // DOM dump via temp file → rename, so we never leave a 0-byte HTML behind.
-  try {
-    const r = spawnSync('browse', ['--ws', target, 'get', 'html', 'body'], { encoding: 'utf8' });
-    if (r.stdout && r.stdout.length) {
-      fs.writeFileSync(tmp, r.stdout);
-      fs.renameSync(tmp, html);
+  // Synchronous calls inside the iteration body cannot be interrupted by
+  // SIGTERM mid-tick: Node only drains the signal callback queue between
+  // event-loop turns, and the only yield point we hit is the awaited
+  // sleep at the bottom. So the iteration body always runs to completion;
+  // SIGTERM responsiveness comes from the per-spawnSync timeout (each
+  // browse call returns within SNAPSHOT_TIMEOUT_MS) plus the abortable
+  // sleep that resolves immediately when the signal arrives.
+  while (!stop.stopping) {
+    const ts   = isoStampForFilename();
+    const png  = path.join(RD, 'screenshots', `${ts}.png`);
+    const html = path.join(RD, 'dom',         `${ts}.html`);
+    const tmp  = `${html}.partial`;
+
+    // Best-effort screenshot. If browse fails we just don't get one this tick.
+    spawnSync('browse', ['--ws', target, 'screenshot', png], {
+      stdio: 'ignore',
+      timeout: SNAPSHOT_TIMEOUT_MS,
+    });
+    if (fs.existsSync(png) && fs.statSync(png).size === 0) {
+      fs.unlinkSync(png);
+    }
+
+    // DOM dump via temp file → rename, so we never leave a 0-byte HTML behind.
+    try {
+      const r = spawnSync('browse', ['--ws', target, 'get', 'html', 'body'], {
+        encoding: 'utf8',
+        timeout: SNAPSHOT_TIMEOUT_MS,
+      });
+      if (r.stdout && r.stdout.length) {
+        fs.writeFileSync(tmp, r.stdout);
+        fs.renameSync(tmp, html);
+      }
+    } catch { /* best-effort */ }
+    // Cleanup any leftover .partial from a previous interrupted iteration.
+    if (fs.existsSync(tmp)) {
+      try { fs.unlinkSync(tmp); } catch {}
     }
-  } catch { /* best-effort */ }
-  // Cleanup any leftover .partial from a previous interrupted iteration.
-  if (fs.existsSync(tmp)) {
-    try { fs.unlinkSync(tmp); } catch {}
-  }
 
-  // URL via the daemon-bypassing one-shot. Returns {"url": "..."}.
-  let urlValue = '';
-  const u = spawnSync('browse', ['--ws', target, '--json', 'get', 'url'], { encoding: 'utf8' });
-  if (u.stdout) {
-    try { urlValue = JSON.parse(u.stdout).url || ''; } catch {}
+    // URL via the daemon-bypassing one-shot. Returns {"url": "..."}.
+    let urlValue = '';
+    const u = spawnSync('browse', ['--ws', target, '--json', 'get', 'url'], {
+      encoding: 'utf8',
+      timeout: SNAPSHOT_TIMEOUT_MS,
+    });
+    if (u.stdout) {
+      try { urlValue = JSON.parse(u.stdout).url || ''; } catch {}
+    }
+
+    const screenshotRel = fs.existsSync(png)  ? `screenshots/${ts}.png` : '';
+    const domRel        = fs.existsSync(html) ? `dom/${ts}.html`        : '';
+    fs.appendFileSync(indexPath,
+      JSON.stringify({ ts, screenshot: screenshotRel, dom: domRel, url: urlValue }) + '\n');
+
+    await stop.sleep(intervalMs);
   }
+}
+
+// ---------------------------------------------------------------------------
 
-  const screenshotRel = fs.existsSync(png)  ? `screenshots/${ts}.png` : '';
-  const domRel        = fs.existsSync(html) ? `dom/${ts}.html`        : '';
-  fs.appendFileSync(indexPath,
-    JSON.stringify({ ts, screenshot: screenshotRel, dom: domRel, url: urlValue }) + '\n');
+// Build a stop signal that listens once for SIGTERM/SIGINT, exposes a
+// boolean view, and provides an abortable sleep so the inter-iteration
+// pause wakes immediately when the user calls stop-capture.
+//
+// Exported as a factory rather than a module-level mutable so a test can
+// drive it deterministically without messing with the live process's
+// signal handlers.
+export function createStopSignal() {
+  let stopping = false;
+  // Pending sleeps register here so `trigger` can resolve them in one
+  // pass. Using an explicit set rather than `stopPromise.then(...)` per
+  // sleep keeps memory bounded: a 24-hour capture at the default 2 s
+  // interval is ~43,000 sleeps, and a chained .then handler closure per
+  // sleep would pin all of them on the long-lived stop promise's
+  // reaction list until SIGTERM finally fired.
+  const pending = new Set();
+
+  const trigger = () => {
+    if (stopping) return;
+    stopping = true;
+    for (const entry of pending) {
+      clearTimeout(entry.timer);
+      entry.resolve();
+    }
+    pending.clear();
+  };
+  process.on('SIGTERM', trigger);
+  process.on('SIGINT',  trigger);
+
+  function sleep(ms) {
+    if (stopping) return Promise.resolve();
+    return new Promise((resolve) => {
+      const entry = { resolve, timer: null };
+      entry.timer = setTimeout(() => {
+        // Self-clean on natural wake-up so the closure becomes
+        // unreachable as soon as the timer fires.
+        pending.delete(entry);
+        resolve();
+      }, ms);
+      pending.add(entry);
+    });
+  }
 
-  await sleepMs(intervalMs);
+  return {
+    get stopping() { return stopping; },
+    sleep,
+    // Test-only entry points. Production code relies on the signal handlers.
+    _trigger: trigger,
+    _pendingCount: () => pending.size,
+  };
 }
diff --git a/skills/browser-trace/scripts/snapshot-loop.test.mjs b/skills/browser-trace/scripts/snapshot-loop.test.mjs
new file mode 100644
index 0000000..2e499a7
--- /dev/null
+++ b/skills/browser-trace/scripts/snapshot-loop.test.mjs
@@ -0,0 +1,88 @@
+// Unit tests for the stop-signal helper used by snapshot-loop.mjs.
+//
+// Run with:  node --test scripts/snapshot-loop.test.mjs
+//
+// The factory exposes `_trigger` so each test can stand in for the live
+// SIGTERM/SIGINT handlers without touching the surrounding process.
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+import { createStopSignal } from './snapshot-loop.mjs';
+
+test('stopping is false until trigger fires', () => {
+  const stop = createStopSignal();
+  assert.equal(stop.stopping, false);
+  stop._trigger();
+  assert.equal(stop.stopping, true);
+});
+
+test('sleep wakes immediately when trigger fires mid-wait', async () => {
+  const stop = createStopSignal();
+  const startedAt = Date.now();
+  const sleeping = stop.sleep(60_000);
+  // Fire the stop on the next tick so the sleep is actually pending.
+  setTimeout(() => stop._trigger(), 5);
+  await sleeping;
+  const waited = Date.now() - startedAt;
+  assert.ok(
+    waited < 1_000,
+    `sleep should have aborted in well under a second, but waited ${waited} ms`,
+  );
+  assert.equal(stop.stopping, true);
+});
+
+test('sleep resolves immediately when trigger has already fired', async () => {
+  const stop = createStopSignal();
+  stop._trigger();
+  const startedAt = Date.now();
+  await stop.sleep(60_000);
+  const waited = Date.now() - startedAt;
+  assert.ok(
+    waited < 100,
+    `sleep should have returned synchronously after stopping, but waited ${waited} ms`,
+  );
+});
+
+test('sleep without a trigger waits for the requested interval', async () => {
+  const stop = createStopSignal();
+  const startedAt = Date.now();
+  await stop.sleep(80);
+  const waited = Date.now() - startedAt;
+  assert.ok(
+    waited >= 70,
+    `sleep should honor its interval when no trigger fires, but waited ${waited} ms`,
+  );
+  assert.equal(stop.stopping, false);
+});
+
+test('repeated triggers are idempotent', () => {
+  const stop = createStopSignal();
+  stop._trigger();
+  stop._trigger();
+  stop._trigger();
+  assert.equal(stop.stopping, true);
+});
+
+test('completed sleeps deregister from the pending set so closures can be GC\'d', async () => {
+  const stop = createStopSignal();
+  assert.equal(stop._pendingCount(), 0);
+  // Run many sleeps back-to-back so a per-sleep .then handler would
+  // accumulate visibly on the stop promise's reaction list.
+  for (let i = 0; i < 200; i++) {
+    const p = stop.sleep(1);
+    assert.equal(stop._pendingCount(), 1, `pending should be 1 mid-sleep on iteration ${i}`);
+    await p;
+    assert.equal(stop._pendingCount(), 0, `pending should drain to 0 after iteration ${i}`);
+  }
+});
+
+test('trigger drains every still-pending sleep without leaking', async () => {
+  const stop = createStopSignal();
+  const sleeps = [stop.sleep(60_000), stop.sleep(60_000), stop.sleep(60_000)];
+  assert.equal(stop._pendingCount(), 3);
+  stop._trigger();
+  assert.equal(stop._pendingCount(), 0);
+  // All three resolve with no further input.
+  await Promise.all(sleeps);
+});
diff --git a/skills/browser-trace/scripts/start-capture.mjs b/skills/browser-trace/scripts/start-capture.mjs
index ab10185..33a533f 100755
--- a/skills/browser-trace/scripts/start-capture.mjs
+++ b/skills/browser-trace/scripts/start-capture.mjs
@@ -65,8 +65,11 @@ const loop = spawn(process.execPath, [loopScript, target, RD, String(interval)],
 loop.unref();
 fs.writeFileSync(path.join(RD, '.loop.pid'), String(loop.pid));
 
-// Give browse cdp a beat to fail loudly on bad targets so the user sees the
-// real error instead of a silent zero-event capture.
+// Give both children a beat to fail loudly on bad targets so the user sees
+// the real error instead of a silent zero-event capture. The snapshot loop
+// is checked too: a syntax error or missing dep there would otherwise leave
+// the run with CDP events but no DOM/screenshots, and the user would only
+// notice after stop-capture.
 await sleepMs(1000);
 if (!isAlive(cdp.pid)) {
   console.error(`browse cdp exited immediately — check ${RD}/cdp/stderr.log`);
@@ -74,6 +77,12 @@ if (!isAlive(cdp.pid)) {
   try { process.kill(loop.pid); } catch {}
   process.exit(1);
 }
+if (!isAlive(loop.pid)) {
+  console.error(`snapshot-loop exited immediately — check ${RD}/snapshot-loop.log`);
+  try { console.error(fs.readFileSync(path.join(RD, 'snapshot-loop.log'), 'utf8')); } catch {}
+  try { process.kill(cdp.pid); } catch {}
+  process.exit(1);
+}
 
 console.log(`run_id=${runId}`);
 console.log(`run_dir=${RD}`);